=20.= To feel absolutely sure that his work is correct is the ambition of every bookkeeper. The fear that an error will throw his books out of balance is always present, and is only dispelled when the ledger is proved at the end of the month. Accountants, bookkeepers, and mathematicians have long searched for an infallible rule or method of checking which would detect an error as soon as it is made. No system could possibly prevent the making of an error, but if detected, an error is quickly corrected. Numerous formulas and checking systems, designed to detect errors in posting, have been put on the market from time to time. Each has been advertised as the only infallible system. All have been eagerly purchased, tried more or less faithfully, and, as a rule, speedily discarded. Modern methods of bookkeeping have done more to aid in the detection of errors in posting than all of the checking systems combined. The sectionalization of accounts not only facilitates the detection of errors, but greatly minimizes the chances of their being made. As an illustration take a sales ledger which is subdivided into two or more parts, with special columns in cash book and sales book, and controlling accounts for each section. If the individual balances taken from the sales ledger do not agree with the controlling accounts, it is seen at a glance in which section the error has been made. Instead of a search through all postings to sales ledger accounts, it is only necessary to check postings to that particular section. [Illustration: Ledger Proof Without a Trial Balance ] Errors in posting to nominal accounts are minimized by special columns in cash book, sales book, and purchase record, with one posting at the end of the month, in place of a posting for each item entered. But still, errors are and always will be made in posting, and for the bookkeeper who has a large number of accounts to post and wishes to keep a constant check on his work, the reverse posting or slip posting system will give satisfactory results with little labor. [Illustration: Fig. 19. Monthly Posting Proof Sheet ] This system is very simple, and easily operated. It consists of the use of a slip of paper with debit and credit columns in which accounts posted to the ledger are entered. A separate slip is used for each book from which postings are made, and the items are posted to the slip from the ledger. The slip should be placed on the desk on the opposite side of the ledger from the book of entry.